A polarizing plate has been usually used in a structure, in which a protective film is stacked on both surfaces of a polarizer formed of a polyvinyl alcohol (hereinafter, referred to as ‘PVA’)-based resin dyed with a dichroic dye or iodine. In this case, a triacetyl cellulose (TAC)-based film has been frequently used as the protective film because the film is excellent in optical transparency or moisture permeability.
Meanwhile, as a liquid crystal display device has recently been developed into a mobile device such as a laptop personal computer, a cellular phone and a car navigation system, it is required that a polarizing plate which constitutes a liquid crystal display device is thin and light weight. However, in a polarizing plate in which a TAC film and the like are laminated as the protective film as described above, it is difficult to maintain a thickness of the protective film at 20 μm or less from the viewpoint of handling property or durability performance during the operation, so that there is a limitation for the liquid crystal device to be thin and light weight.
In order to solve the aforementioned problems, a technology has been suggested, in which a protective film is provided on only one surface of a polarizer, and a transparent thin film layer is formed by coating an active energy ray curable composition on the other surface thereof. Meanwhile, the active energy ray curable composition which has been suggested may be classified into a radical curable composition and a cationic curable composition according to the curing method. In this case, when a transparent thin film layer is formed using a cationic curable composition, there is an advantage in that excellent adhesion with the polarizer is achieved, but there are many disadvantages in the manufacturing process due to a slow curing rate and a low degree of curing.
In order to solve such problems of the cationic curable composition, a technology has been suggested, in which a transparent thin film layer is formed by using a radical curable composition which includes an acrylic compound as a main component. However, in the case of a radical curable composition including an acrylic compound as a main component, there are problems in that the curing rate is fast compared to a cationic curable composition, while the glass transition temperature of a protective layer after curing of the radical is low, such that heat resistance reliability of a polarizing plate to be manufactured using the same is not good.
In order to solve the problem, there has been suggested a technology of additionally adding a hydrophilic and monofunctional monomer to a radical curable composition and using the monomer in mixture with the acrylic compound in order to increase the glass transition temperature of the radical curable composition. However, when these are additionally added, the glass transition temperature of the radical curable composition may be increased, but since there is an aspect in which water resistance which a polarizing plate needs to basically have becomes considerably reduced, the addition of these compounds has been fatally disadvantageous in application to the polarizing plate.
Accordingly, there is a need for a new polarizing plate which is excellent in water resistance and heat resistance while having a protective layer which may be formed by radical curing.